Our transcription: Unconformities, the third great class of geologic structure are not as useful as folds and faults in analyzing past crustal stress. Nonetheless, they too have proven to be an important key to the past. Just as astronomers are preoccupied with the immensity of space, geologists are uniquely fascinated with time and Earth history. The first geologist to actually recognize the scale of geologic time was a Scotish intellectual named James Hutton. Over a century ago, Hutton recognized that a sequence of layered rocks is a physical record of some portion of Earth history. He also predicted that in places where mountain building has occurred some part of that record would be destroyed by erosion. Armed with this hypothesis, Hutton found places where old rocks had been covered by much younger sedimentary rocks. A contact between these two rock formations marked a period of Earth history with no rock record. This geologic structure is called an unconformity.